How long to let a car cool down before working on the cooling system?

One hour? Two?

Car in question is an '11 Mustang GT. I need to R&R the degas bottle, so I'm not working on the radiator, but I do need to open the pressure cap ("radiator cap" ) in order to do the work.

Until the cap pressure doesn't bubble the coolant out of the bottle. Probably about 30 min to 1hr.

When theres no pressure in the upper hose its usually safe.

I usually go by the upper hose squeeze test. Once the system is de-pressurized, it's safe to open it up. Of course, it can still be a little hot for comfort for some jobs...

The length of time depends upon the heat transfer rate. If its cold outside the system will naturally cool down more quickly than if its 100 deg F. For safe temperatures see this website. https://www.pseg.com/home/education_safety/safety/scalding.jsp

How hot is your home's hot water that you can stand?

Wait until the coolant is that temp.

Thanks, guys. I'll wait about an hour or so and then see how hot it is. I don't even need to touch the coolant in order to do the swap, but I don't like depressurizing the system using the radiator cap.

The big thing is the system can't be hotter than the atmospheric boiling point of the coolant, or you'll get a steam explosion out the filler neck when you let the pressure off.

BrokenYugo wrote: The big thing is the system can't be hotter than the atmospheric boiling point of the coolant, or you'll get a steam explosion out the filler neck when you let the pressure off.

This. It doesn't take long too cool below that point. I can't imagine more than 10 mins or so. If you have a cap that threads on (not 1/4 turn), you can just loosen the threads gently and listen for hissing.

It depends. Here's a helpful flowchart:

Are you in the middle of a race?
...|_ No - Go grab a taco and let it chill for a half hour or so.

...|_ Yes - Man up nancy, grab some rags the cap and depressurize it! You've got a head gasket to swap, chuckles!

Longer than I waited one time on the side of I-64 in West Virginia. My right shoulder and a few other spots blistered nicely from the jet of escaping coolant. Stupid hurts.

If in a hurry, open hood and aim a box fan at the front of the car.

Otherwise, just take your time till its meerly warm.

(you are an engineer, you know how newtons law of cooling works )

I've never really had an issue opening systems hot. Use rags, open it slowly, understand there is a fair chance of getting severely burned.

Also understand, that there is a possibility of all the coolant in the block instantly flashing to steam and erupting out of the cap, the instant the pressure is released. If you loosen the cap and suddenly hear bubbling, tighten the cap back down and walk away for a while. This is very important to remember on a overheated engine.

I always found that floor mats work wonders for keeping hot coolant away from your skin if you absolutely positively have to open a system that is hot and under pressure

I usually upgrade to a lever-vent cap. That way you can vent it without opening the cap. That only works on cars with the pressure cap on the rad though.

But if your pressure cap is on the surge tank, you can usually do it right away. On the radiator, the cap is directly in contact with the coolant and you don't get a warning before you get sprayed. If its on the surge tank, you get a few seconds of air first.

As long as its not overheating, you should be fine at any time, but always expect some overflow as things are expanded and under pressure

But the real answer to your question is hard to say. I can let my F150 sit for about 4 hours and the heater still blows warm when I start it up again. My ex wife's Scion xB you get about an hour before its bone cold again.

Toyman01 wrote: I've never really had an issue opening systems hot. Use rags, open it slowly, understand there is a fair chance of getting severely burned.

This is how you do it!

+1 on the box fan. Open the hood, lay it on top of the engine, and go have lunch. It'll be stone cold by the time you get back.

Thought I'd post my technique for opening a hot system that I had to develop in a 24-hour offroad rally where the engine was burning off enough coolant that I had to top it up every few hours to prevent an overheat. This is for the common push-and-turn type of radiator cap but the technique with a threaded cap would be pretty much the same.

Warning, this will cause burning hot toxic coolant to spray out, just not onto you if you do it properly.

Stand as far from the radiator as you can, and as far off to the side (vs. directly in front of the car) as you can, with one hand slowly turn the cap. You don't want to turn it to the fully released position where it could be lifted off. As soon as you hear hissing or bubbling or the pop of the pressure being released, let go and get back, that means the coolant is about to flash-boil and erupt. Because the cap is still on, the coolant will spray downward in a cone shape from the underside of the cap. A lot of it will come out. Once it stops spraying, you can remove the cap completely. Try not to inhale the steam.

dropstep wrote: When theres no pressure in the upper hose its usually safe.

This is my answer. If I can grab the upper hose and hang on to it without burning myself I go to step two. Give it a squeeze. if you can squeeze it, and hear the fluid flow in it and it doesn't "force" you to relax your grip. You should be ok. Still use a towel and only open it to the first notch till you're absolutely sure it's not pressurized.

When you can put your tongue on top radiator tank without flinching?